Continuous multistage thermophilic aerobic sludge digestion system

ABSTRACT

A continuous flow multistage aerobic wastewater sludge digestion system comprising at least two stages connected in series, each stage comprising one or more covered tanks, tank segments or compartments, each having an enclosed separate gas headspace, wherein the gas headspaces of each stage are connected in a specified sequence that may be identical to or different from the staging sequence followed by the sludge liquid. An oxygen containing gas stream, such as air or an oxygen enriched gas stream, is supplied to the headspace of each stage, and a means for aerating the sludge in one or more of the tanks, tank segments or compartments is provided to enhance the transfer of oxygen from the gas headspace into the sludge liquid for use by bacteria in digestion of the sludge. At least one stage of the sludge digestion system is operated at a thermophilic temperature above 50° C. and the flow of oxygen containing gas through the system can be cocurrent, countercurrent, or a mixture of both with respect to the flow of the liquid.

FIELD OF THE INVENTION

The present invention relates generally to systems for aerobicdigestion, pasteurization and stabilization of wastewater sludges. Moreparticularly, the invention relates to a continuous multistage,autothermal thermophilic aerobic digestion (ATAD) system for digesting,stabilizing and pasteurizing wastewater sludges as well as achievingsignificant reduction in the quantity of sludge requiring disposal.

BACKGROUND OF THE INVENTION

The need for human wastewater collection and treatment has beenrecognized for centuries. Initially, this need was driven by the desireto reduce disease caused by humans living in close proximity to theirwaste, but more recently wastewater treatment methods have evolved witha desire to reduce or eliminate water pollution and achieve desiredlevels of water quality. In the United States in the 1800s, the firstmajor evolution of wastewater disposal began when pit privies and opendrainage ditches were replaced by buried sewers that transmitted wastesand stormwater to other locations where it would have less effect on thecommunity. The sewered population rapidly increased from about 1 millionin 1860 to about 25 million in 1947 reflecting public awareness of thelink between human disease and waste disposal practices.

Once large quantities of wastewater began being collected by sewers, itbecame possible to develop treatment processes to reduce or eliminatethe harmful effects of sewerage on human health and the environment. Thefirst treatment methods were basically anaerobic processes wherecollected sewage was allowed to digest and stabilize essentiallyundisturbed. However, near the end of the 1800s several researchers,with the idea that aerobic treatment would avoid malodorous anaerobicconditions and undesirable results, began to explore blowing air intosewage tanks. Over the course of the next three decades, experiments inaerobic treatment of sewerage led to the conclusion that aeratingwastewater in the presence of a suspended biomass (achieved throughsolids recycle) was a very effective method of treating wastewater todegrade the biological constituents in the wastewater. In 1914 thisprocess was coined the “activated sludge process” and has since becomethe standard method for secondary wastewater treatment.

The activated sludge process is a biochemical type of reaction. Itinvolves the mass transfer of oxygen from an oxygen containing gas intothe wastewater and then the mixing and use of that dissolved oxygen tosupport the growth of aerobic microorganisms suspended in thewastewater. These microorganisms, known as the biomass, oxidize theorganic materials in the wastewater in different ways to eliminate thebiochemical oxygen demand of the wastewater. FIG. 1 depicts a simpleschematic diagram of a typical, flow-through, modern activated sludgeprocess for “secondary” wastewater treatment. Effluent from “primary”treatment which typically involves just grinding and settling in aprimary clarifier is the influent 15 to secondary treatment. The primaryeffluent 15 and recycle biological solids 16 (activated sludge) arefirst combined as the influent and are mixed and aerated in abiochemical reactor 17. Oxygen necessary for the process is provided byair or oxygen enriched gas 20 and aeration is usually hastened by use ofgas-liquid contacting devices such as diffusers, surface aerators, andsparging mixing impellers (not shown). Usually the process operates in acontinuous-flow mode, but can also be operated as a batch process.Contents of the reactor, referred to as mixed liquor, consist ofwastewater, microorganisms (living and dead), and inert, biodegradable,and non-biodegradable suspended and colloidal matter. The particulatesolid fraction of the mixed liquor is termed mixed liquor suspendedsolids (MLSS).

After a sufficient residence time for the biological reactions to occur,the mixed liquor is typically transferred to a separate settling basin18 (clarifier) to allow gravity separation of the MLSS from the treatedwastewater. Settled MLSS is then recycled to the aeration/reactor basinas recycled sludge 16 to maintain a sufficiently concentrated microbialpopulation for rapid degradation of the influent wastewaterconstituents. Because there is usually a significant net positiveproduction of biological MLSS (the rate of cell synthesis exceeds therate of cell destruction) an increasing inventory of sludge builds up inthe system and the excess activated sludge 19 must be discarded or“wasted” from the process continuously or periodically. Wastingtypically is from the secondary gravity clarifier or sludge recyclestream, but direct removal from the aeration basin or reactor is also analternative. The final result of the activated sludge process is twoseparate streams: the treated effluent wastewater 20 and the excesswaste activated sludge stream 19. The effluent is a liquid/water streamvery low in solids content and soluble pollutants that is sometimestreated to further improve its water quality prior to discharging itback into the environment. Further treatment options for the activatedsludge treated wastewater effluent include nutrient removal processesand sterilization through ozonation or by UV radiation.

The waste activated sludge stream from the secondary wastewatertreatment process is relatively high (compared to influent wastewater)in solids content (e.g. 1–3 wt % total solids) and is also typicallyfurther treated. The waste activated sludge is often combined withsludge from a primary clarifier operating in front of the activatedsludge process. It is highly desirable to process this waste sludge insuch a manner that it can be readily and economically disposed ofwithout creating further pollution of the ecosphere. Further wastewatersludge treatment usually leads to either a concentrated liquid that canbe land applied as a soil reconditioner, a stabilized solid biomass thatis landfilled, or a pasteurized biosolid that can be beneficially usedin some manner such as for a fertilizer or as a composting material.

The basic aim of all wastewater sludge treatment processes is toeconomically and efficiently reduce and stabilize waste sludge solids.In addition, the sludge treatment system should desirably also producean end product which is fully suitable for final disposal withoutfurther physical or chemical treatment. In conventional practice finalsludge disposal is commonly carried out by incineration, land filling orland spreading. In many instances, land disposal is employed and isparticularly attractive due to minimal long-term environmental effectsand is highly advantageous in contributing to reconditioning of thesoil. However, the use of land spreading as a final sludge disposalmethod requires a well stabilized and pasteurized end product, so thatthe concentration of pathogenic organisms in the sludge is sufficientlylow to avoid a potential health hazard in disposal of the sludge and thesludge is adequately stabilized to prevent further degradation in theenvironment.

Traditionally, three distinct processes have been widely utilized fortreating wastewater sludge: oxidation ponds, anaerobic digestion andaerobic digestion. Oxidation ponds are generally employed in the form ofcomparatively shallow excavated earthen basins which extend over a largearea of land and retain wastewater prior to its final disposal. Suchponds permit the biological oxidation of organic material by natural orartificially accelerated transfer of oxygen to the pond water from theambient air. During the bio-oxidation process, the solids in thewastewater are biologically degraded to some extent and ultimatelysettle to the bottom of the pond, where they may become anaerobic andare further stabilized. Periodically the pond must be drained and thesettled sludge dredged out to renew the volumetric capacity of the pondfor further wastewater sludge treatment, and the withdrawn sludge isutilized for example as landfill. Oxidation ponds thus represent afunctionally simple system for wastewater sludge treatment. The use ofoxidation ponds, however, has limited utility, since their operationrequires sizable land areas. Moreover, no significant reduction of thelevel of pathogens in the sludge is accomplished by this elementarytreatment and disposal method.

Anaerobic digestion has generally been the most extensively usedwastewater sludge digestion process for stabilizing concentrated organicsolids, such as are removed from settling tanks, biological filters andactivated sludge plants as discussed above. In common practice, theexcess waste sludge is accumulated in large covered digesters where thesludge is mixed and naturally fermented anaerobically for about 30 days.The major reasons for widespread commercial use of anaerobic sludgedigestion are that this method is: (1) capable of stabilizing largevolumes of dilute organic slurries, (2) results in significantbiological solids (biomass) reduction and stabilization, (3) produces arelatively easily dewaterable sludge, (4) a net producer of methane gas,and (5) potentially capable of producing a pasteurized sludge under theright conditions. Anaerobic digestion is characteristically carried outin large scale tanks which are more or less thoroughly mixed, either bymechanical means or by the recirculation of compressed digester gas.Such mixing rapidly increases the rate of the sludge stabilizationreactions by creating a large zone of active decomposition.

Methane gas is produced during anaerobic digestion and ischaracteristically used in combustion heaters to offset heat losses ofthe anaerobic digestion process which usually operates at above ambienttemperatures. However, seasonal temperature variations and fluctuationsin the suspended solids level of the influent wastewater sludge have asignificant effect on both the rate of methane gas production and theamount of heating which is necessary to maintain the digestion zone atthe desired elevated temperature operating level. As a result, ifelevated temperature conditions are to be maintained year round in theanaerobic digestion zone, an auxiliary heating system is generally anessential element of the overall sludge digestion system.

Since the rates of anaerobic digestion and resultant methane gasformation are strongly influenced by the suspended solids content of thesludge undergoing treatment and by the temperature level in thedigestion zone, it is in general desirable to feed as concentrated asludge as possible to the digester, thereby minimizing heat losses inthe effluent stabilized sludge stream discharged from the anaerobicdigester while maximizing methane production in the digester. However,even with such provisions elevated temperatures are difficult tomaintain economically in the anaerobic digestion zone, especially duringwinter months. Furthermore, even comparatively small temperaturefluctuations in the anaerobic digestion zone may result indisproportionately severe process upsetting and souring of the digestercontents, as is well known. Perhaps the most important disadvantage ofanaerobic sludge digestion systems is the requirement for largeresidence times of about 30 days that are needed to achieve adequatestabilization. These large residence times result in very large tankneeds and correspondingly large capital costs for tank construction andmixing.

As an alternative to the foregoing anaerobic methods, biodegradablewastewater sludge can be digested aerobically. Air and to a much lesserextent high purity oxygen has commonly been employed in practice as thesource of oxygen for this purpose. It is well known that aerobicdigestion proceeds more rapidly at elevated temperatures. As temperaturerises above 40° C., the population of mesophilic microorganisms declinesand thermophilic forms increase. The temperature range of about 50–70°C. is often referred to as the thermophilic range where thermophilicbacteria predominate and where most mesophils are extinct. Above thisrange, the thermophils decline, and at 90° C., the system becomesessentially sterile. Because of the more rapid oxidation of sludgebiomass at higher temperatures, thermophilic digestion achieves morecomplete removal of biodegradable volatile suspended solids (BVSS) thanthe same period of digestion at lower temperatures. A more stableresidue is obtained which can be disposed of without nuisance.Thermophilic digestion can also effectively reduce or eliminatepathogenic bacteria in the sludge (pasteurize the sludge), therebyavoiding the potential health hazard associated with its disposal.

When air systems are used to supply dissolved oxygen for aerobic sludgedigestion systems, with the air being passed through the body of sludgeliquid in a digestion tank and freely vented to the atmosphere, the lossof heat from the sludge to the air being passed through the digestertends to become substantial in magnitude. This loss of heat is due inpart to the sensible gas temperature heat loss of the hot gas beingdischarged into the atmosphere, but more importantly due to theevaporative heat loss of the gas caused by the evaporation ofsubstantial quantities of water into the gas phase during the oxygendissolution process. The air being contacted with the higher temperaturesludge biomass will quickly come to the temperature of the sludgebiomass being aerated and will also rapidly evaporate enough water intothe gas phase to quickly bring the water content of the gas phase intogas-liquid equilibrium with the sludge liquid from the standpoint of thewater vapor content of the aeration gas. As a result, aerobic digestionin the past has often involved digestion with only mesophilicmicroorganisms. However, more recently air sludge digestion systemsoperating in the thermophilic temperature range have become more commonby employing such techniques as covered and highly insulated tanks,external heat sources and heat exchange equipment to minimize both thegas phase and liquid phase heat losses from the aerobic digestercontents. Air contains only 21% oxygen and only about 10–20% of theoxygen content thereof is dissolved and available to the bacteria in theaerobic sludge digestion system. Accordingly, a very large quantity ofair must be used to supply the oxygen requirements of the process andthe heat losses from the digester associated with venting the sensibleheat of the “spent” air and the latent heat required to saturate thespent air with water vapor are substantial. As a result of these heatlosses in conventional air aerobic sludge digestion systems, very largequantities of external heat and/or extensive heat transfer equipmentmust be employed to sustain the sludge temperatures at the elevatedthermophilic levels.

Several strategies have been employed to avoid the need for the additionof external heat into thermophilic aerobic sludge digestion systems.These methods are generally classified as autothermal aerobic digestionsystems or “ATADs”. The ATAD process is an aerobic digestion processthat achieves thermophilic operating temperatures without externalsupplemental heat beyond that supplied by the aeration and mixingenergy. Within the ATAD bioreactor, sufficient levels of dissolvedoxygen, volatile solids, and mixing allow aerobic microorganisms todegrade organics to carbon dioxide, water, and nitrogen byproducts,during which significant heat energy is released and absorbed into theliquid phase. If sufficient insulation, residence time, and adequatesolids concentrations are provided, the process can be operated atthermophilic temperatures to achieve a high level of volatile solidsdestruction and pathogen reduction sufficient to meet U.S. EPAregulations for the 40 CFR Part 503 Class A designation.

Since the early 1980s the U.S. EPA has promoted the use of biosolids inagriculture and issuance of the 40 CFR 503 regulations in 1993 furtherencouraged the practice. These regulations require that any biosolidsapplied to land must meet certain pathogen and vector attractionreduction limits. For example, the Class A designation specified in theregulation requires that pathogen levels have been reduced to belowdetectable levels. The regulations provide for six alternatives formeeting the pathogen reduction requirements. As an example, onealternative is to ensure that all particles are processed for a timedetermined by the following equation: D=50,070,000/10^(0.14t) whichapplies when total solids are <7%; t is ≧50° C.; and D is residence timewhich is ≧30 minutes. The second requirement of the regulations isrelated to stabilization or vector attraction reduction. The regulationsgive at least 10 options for meeting vector attraction reduction. Oneexample is a 38% reduction in the volatile solids component of thesludge.

Air ATADs have been known for about two to three decades. FIG. 2 shows aschematic for a conventional type of Air ATAD system. Liquid feed sludge22 is first thickened in a thickener 23 to at least about 3% solidsbefore entering one or more of the ATAD reactors 24. The reactors aretypically enclosed and insulated. They also include mixing, aeration, 25and foam suppression equipment and are operated in batch mode with asludge retention time of from about 5–10 days. These ATADs typicallyoperate with two tanks or bioreactors in series but are not operated ina continuous flow manner. Some stabilization and heating occur in thefirst tank, with further stabilization and heating to temperatures ofabout 55° C. to 65° C. occurring in the second tank. Feeding is oftenintermittent, with removal of digested solids from the second tank,transfer of digesting solids from the first to the second tank, andaddition of feed solids to the first tank. This promotes temperatureelevation and minimizes short-circuiting of feed solids to thestabilized solids, thereby enhancing pathogen destruction. Liquid exitsto a storage or cooling tank 26 before being further processed and/orland applied. The exit gas (offgas) 27 is vented or further treated suchas by scrubbing. Benefits of ATAD include a high disinfectioncapability, relatively low space and tankage requirements, and a highsludge treatment rate. It is an effective and environmentallyresponsible means of achieving aerobic stabilization and producingsludge that meets the current regulations for Class A sludge pathogencontrol and for disposal of agricultural, municipal and industrialwastewater sludge on land and underground.

Single tank ATAD systems are also known that operate with a feedingtechnique called a partial fill and draw process where for example on adaily basis partial withdrawal from the reactor of about 1 days volumeof sludge will occur for about 1 hour, then new feed will fill the tankback up followed by batch reacting for the remainder of the 24 hourcycle. This method limits the temperature swing of the system, butrequires higher tank volumes. Digested sludge withdrawn from an ATAD canbe further processed using conventional techniques such as dewateringprior to final disposal.

It is also known that heat losses in aerobic sludge digestion systemscan be reduced by using oxygen-enriched or high purity oxygen gas ratherthan air. If a high utilization of the high purity oxygen gas can beachieved, the total amount of gas which must be fed to and vented fromthe aerobic digester is considerably smaller compared to air, becausemost if not all of the inert nitrogen gas has been removed. Heat lossesdue to sensible warmup and to water evaporation into the high purityoxygen gas stream are also significantly decreased. These reductions inheat losses are sufficient for autothermal heat alone to sustain thetemperature at levels appreciably higher than ambient, so that thedigestion zone is able to operate efficiently in the thermophilictemperature regime with no input of external heat to the process. Sincethermophilic stabilization is much more rapid than mesophilicstabilization, the necessary residence time to achieve adequatestabilization in the aerobic digestion zone is also greatly reduced inthe thermophilic mode. This in turn permits the use of smaller reactorswhich further reduces heat losses to the surroundings. Because of thefaster rate of oxidation of sludge, oxygen ATAD can achieve suitablyhigh biodegradable volatile solids reduction, in comparatively shortsludge retention periods.

Despite their significant attractive features, ATAD systems have severalassociated disadvantages relative to anaerobic sludge digestion. First,since the thermophilic aerobic digestion process is oxidative incharacter, the process produces a bio-oxidation reaction product gascontaining carbon dioxide and water vapor which have no end use utilityand are directly vented to the atmosphere or scrubbed. By contrast,anaerobic digestion produces methane gas as a reaction by-product whichmay be exported from the treatment facility and is also useful as a fuelgas for satisfying the heating energy requirements associated withdigestion at elevated temperatures. In addition, the aerobic digestionzone requires a much greater energy expenditure, for mixing andgas-sludge contacting, than is required in the anaerobic digestionsystem for mixing of the digester contents.

Many United States patents have been issued for improved aerobic sludgetreatment processes operating in the thermophilic temperature range.Some representative examples include: U.S. Pat. No. 3,745,113 to Fuchs,U.S. Pat. No. 4,246,099 to Gould et al., U.S. Pat. No. 4,277,342 toHayes et al., U.S. Pat. Nos. 4,975,194 and 4,983,298 to Fuchs et al.,U.S. Pat. No. 5,587,081 to Norcross et al., U.S. Pat. No. 5,948,261 toPressley, U.S. Pat. No. 6,068,047 to Buchhave, U.S. Pat. No. 6,203,701to Pressley et al., and U.S. Pat. No. 6,325,935 to Hojsgaard. Several ofthese processes employ autothermal thermophilic aerobic digestion, or“ATAD”, technology to treat the sludge biomass.

While existing aerobic sludge digestion systems provide viable sludgeutilization and disposal alternatives, they also have a number oflimitations. Most notably, many systems are not a reliable andpredictable means of producing a pasteurized (Class A) sludge that hasbeneficial environmental uses. Moreover, operational difficulties existwith some conventional ATADs, particularly because they are mechanicallymore complex, require larger tanks, require expensive heat transferequipment, and/or are subject to severe foaming.

Over the years, many solutions have been proposed for improving thedisposal of wastewater sludge and overcoming the limitations of ATAD. Asevidenced by the variety of patents mentioned above, there continues tobe a need for further improved designs. Thus, while much effort has beenspent in development of improvements in sludge treatment technology aswell as in refinement of existing sludge treatment processes, therestill exists a great need for better and more efficient and effectivesludge treatment and disposal systems. There is especially a need for anefficient, aerobic sludge treatment system that is capable of producinga Class A pasteurized sludge at lower operating and capital costs. Theseare the primary needs addressed by the present invention.

Accordingly, it is an object of the present invention to provide animproved process for aerobic thermophilic digestion of wastewatersludge.

It is also an object of the invention to provide an efficient multistageaerobic sludge digestion system that is operated in a simple to operateand reliable performance continuous flow manner.

It is also an object of the present invention to provide an aerobicsludge digestion system with improved efficiency and lower operatingcosts and/or lower capital costs compared to current systems.

It is a further object of the invention to provide an aerobic sludgedigestion system that can destroy pathogenic organisms and organicmatter within wastewater treatment sludge so as to reliably produce aClass A pasteurized and adequately stabilized sludge.

It is a further object of the invention to provide an aerobic sludgedigestion system that can be operated autothermally in the thermophilictemperature range at significantly lower capital and energy costswithout the need for external heat sources or heat exchangers.

It is another object of the invention to provide an efficient multistagethermophilic aerobic sludge digestion system that can be integrated infront of an existing anaerobic sludge digestion process.

It is another object of the invention to provide an aerobic thermophilicsludge digestion process employing aerobic digestion and anaerobicdigestion at elevated temperature, in a manner which utilizes theadvantages of each while minimizing their attendant disadvantages.

It is a further object of the invention to provide a multistage sludgedigestion system with the ability to specifically select the gas toliquid contacting staging order to optimize the overall performance ofthe entire system.

Other objects and advantages of this invention will be apparent from theensuing disclosure and appended claims.

SUMMARY OF THE INVENTION

The subject invention is a continuous, multistage, aerobic wastewatersludge digestion, pasteurization, and stabilization system, comprising:(1) at least two stages connected in series, each stage comprising oneor more covered tanks, tank segments or compartments, each having anenclosed separate gas headspace, (2) wherein the gas headspaces of eachstage are connected in a specified sequence that may be identical to ordifferent from the staging sequence followed by the sludge liquid, (3)an oxygen containing gas, such as air or an oxygen enriched gas stream,fed to the gas headspace of one or more of the tanks, tank segments orcompartments, and (4) surface aerators for aerating and mixing thesludge liquid in the tanks, tank segments or compartments to enhancetransfer of oxygen into the sludge liquid biomass for use by thebacteria in sludge digestion and stabilization. According to theinvention, at least one stage of the aerobic sludge digestion system isoperated in the thermophilic temperature range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic of a typical, flow-through, modern activatedsludge system.

FIG. 2 shows a schematic for a conventional type of air autothermalaerobic sludge digestion system (ATAD).

FIGS. 3A and 3B show diagrams illustrating two embodiments of thecontinuous multistage aerobic sludge digestion system of the presentinvention.

FIG. 4 shows a diagram of another embodiment of the present withoptional anaerobic digester and optional alternating batchpasteurization tanks.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a continuous flow system for the autothermal,thermophilic, aerobic digestion of municipal and/or industrialwastewater sludge and is referred to herein as a “continuous multistageautothermophilic aerobic digester” or “continuous multistage ATAD”.Essentially, the system produces a pasteurized and stabilized sludgereferred to as a “Class A” sludge by EPA regulation 40 CFR 503. Thesystem and process of the invention comprises: (1) at least twocontinuously operated stages (at least three in some embodiments)connected in series, said stages employing one or more covered tanks ortank segments and containing liquid sludge which enters the system atthe first liquid stage and exits at the last liquid stage; (2) eachstage having an enclosed separate gas headspace defined by the spaceabove the desired sludge liquid level, wherein the gas headspaces ofeach stage are interconnected in a specified sequence that is identicalto or different from the staging sequence followed by the liquid; (3) anoxygen containing feed gas supplied to the gas headspace of one or moreof the tanks or tank segments as one or more feed streams and flowingthrough at least one tank or tank segment and exiting at one or moreexit streams; and (4) means for providing vigorous gas-liquid contactand sludge liquid mixing in one or more of the tanks or tank segments orcompartments to enhance transfer of oxygen into the sludge liquid foruse by the bacteria in sludge digestion.

The sludge feed stream to the continuous multistage aerobic digester ofthe present invention can be practically any industrial or municipalwastewater sludge stream containing greater than about two percent totalsuspended solids (TSS). Generally, the sludge being treated is producedby an activated sludge plant and constitutes either primary sludge,secondary sludge or a combination of both. In such a plant, the primaryand secondary sludges are often combined and after being combined, arepumped, or otherwise transported, to a thickening station forconcentrating the biomass to a higher solids concentration. Thethickening station may comprise, for example, a horizontal solidbowl-decanting centrifuge, gravity belt filter, dissolved air flotation,gravity settling, evaporative heat treatment, or mechanical drumthickener that is capable of removing water from the sludge andincreasing the TSS content to at least about 3 wt % and usually to about4–5 wt % TSS.

From the thickening station, the sludge is then pumped to an optionalholding tank (useful to achieve flow equalization) or directly into thefirst stage of the continuous flow multistage aerobic sludge digestionsystem via a feed line or similar piping apparatus represented as inflow1 in the schematic diagram of FIG. 3. Depending upon the specificcircumstances such as: (1) the total volume of sludge to be treated, (2)existing plant facilities, and (3) costs, the multiple stages of theinvention can be separate tanks, common wall tanks, or a segmented orcompartmentalized single tank. Additionally, the materials ofconstruction are not critical with steel and fiberglass cylindrical andconcrete common wall rectangular tanks being the typical choices. One ofthe most important features of the reactors, whatever theirconstruction, is that they have individual and enclosed gas headspacesfor each stage. Separate headspaces allow each reactor to have its owngas stage and permits gas staging to increase the performance and/orefficiency of the overall multistage system. This is a key feature ofthe invention not recognized by the prior art.

As is known to those skilled in the art, flow control valves could, ifnecessary, be operatively connected to the respective tanks to controlthe flow of sludge to and from the tanks. If desired, a controller maybe used to operate the flow control valves in a preselected and timedsequence. However, in a preferred embodiment of the invention, thestages will be rectangular in cross section with common wallconstruction between as many stages as possible to eliminate the needfor multistage control valves. Each tank or stage must be properlyinsulated to provide efficient conservation of heat. Examples ofsuitable insulation materials include polyurethane foam and polystyreneat thicknesses of from about 3 cm to 10 cm and other materials andmethods known to those in the industry. Each of the reactors may alsocontain common sensors for record keeping or control purposes includingliquid temperature and dissolved oxygen sensors.

Each tank or stage also includes at least one opening for allowingoxygen containing gas to be introduced from outside the tank to the gasspace or “headspace” thereof, defined as the enclosed region of the tankabove the surface of the sludge liquid. At least one stage is connectedto one or more gas blowers which blow air, oxygen enriched air,substantially pure oxygen or other oxygen containing gas into the stagegas headspace. In the case of physically separate liquid stage reactors,gas distribution pipes are used to interconnect the gas headspaces ofeach tank. In the case of common walled tanks or segmented tanks, thegas flow between stages can simply be provided by suitable openings inthe headspace area of the common walls between stages. Additionally, airflow meters and/or oxygen gas analyzers may be employed at variouspoints throughout the multistage system.

The number and size of the reactors or stages of the invention can beadjusted in accordance with desired operating conditions and feed sludgecharacteristics. Increasing the size or volume of a particular reactoror stage has two primary effects: it increases the sludge residence timeof that particular stage and increases the residence time of the overallsludge digestion system. Residence time is an important design criteriaand can affect the multistage temperature profile and the overall totalsolids digestion in the system. Total sludge residence times of from 2–6(typically 3–6) days are typical of the aerobic systems of the presentinvention. Even shorter residence times (approximately 1–3, preferablyless than 2.5 days) will be typical with embodiments of the inventioncomprising both aerobic and anaerobic treatment systems.

The total number of stages in the continuous multistage thermophilicaerobic digestion system can vary widely. A minimum of two aerobicstages is required when the last stage of the system is a thermophilicstage or when the aerobic system is a precursor to an anaerobicdigester. A minimum of three aerobic stages is required when the laststage is operating in the mesophilic temperature range. For purposes ofthis invention, thermophilic temperatures are in the range of about50–70° C. and mesophilic temperatures are in the range of about 30–50°C. The number of stages is typically from 3–10, often from 4–9 and in atleast one embodiment of the invention is preferably at least 5. Themultiple continuous stages of the invention provide significantlyimproved performance over the prior art by increasing the overallvolumetric efficiency of the sludge digestion process and greatlyreducing the gas phase heat loss from the system.

The oxygen containing gas of the invention preferably contains greaterthan 20 percent by volume oxygen. The oxygen containing gas can be air,oxygen enriched gas or high purity oxygen. As defined herein, “highpurity” means an oxygen containing gas having an oxygen concentrationgreater than about 80% pure oxygen by volume, more preferably greaterthan about 90% pure oxygen by volume, and most preferably greater thanabout 95% pure oxygen by volume. In some embodiments of the invention,the use of high purity oxygen is preferred to enhance the rate of oxygendissolution into the sludge and thereby promote the rate of autothermalheating thereof. This is particularly the case when the sludge has a lowbiodegradable volatile solids (BVSS) content which is typically the casewith sludge of less than about 3 wt % TSS or less than about 1.5 wt %BVSS since BVSS is often around one-half of the TSS content in municipalwastewater sludge. Sludge of about 2 wt % TSS and about 1% BVSS isconsidered very low in BVSS content. However, as will be more fullydiscussed in the examples below, air is the preferred gas in otherembodiments of the invention.

It is particularly important to note that the flow of oxygen containinggas through the multistage system, the “gas staging”, is a criticaldesign variable that can vary from one embodiment to another. Inaddition to the options of being completely cocurrent or completelycountercurrent, the flow of gas may also be partially cocurrent and/orpartially countercurrent relative to the flow of the liquid sludge. Theability to specifically provide the optimal gas staging in order tooptimize the overall performance of the entire multistage system is akey operational feature that distinguishes our system over prior artprocesses. In one embodiment the order of gas staging is determined bythat which minimizes overall electrical power requirements to produceClass A sludge. In another embodiment, the order of gas staging isdetermined by that which maximizes BVSS reduction. In yet anotherembodiment, gas is introduced into the system at a stage other than theinitial or final sludge stages such that gas enters the highesttemperature stage and exits the lowest temperature stage. In a preferredembodiment using air as the feed gas, the number of stages is at leastfive, the gas flows countercurrent to the liquid, the last sludge stageis mesophilic, and the system has at least two thermophilic stages. Inyet a further preferred embodiment the first sludge stage is alsooperated in the mesophilic temperature range. Additionally, it is anoption of the present invention to employ multiple gas feed streams intoseparate stages. For example, a separate air feed stream may be providedin one or more of the final stages to increase the evaporative coolingeffect of the gas flow.

The supply or feeding of the oxygen containing gas to the multistagegas-liquid contacting system in the case of air aeration is quite simplyaccomplished by a low pressure air blower since the gas headspaces inthe entire system will be only a few inches of water pressure above theambient atmospheric pressure. The flow of gas between successive gasstage headspaces following the feed gas stage headspace supplied by theabove blower is accomplished through appropriate sized openings in thecommon walls between successive stages or by suitable conduit means ifthe stages are in totally separate physical containers. The powerrequirement of the low pressure air feed gas blower is relatively smallsince the entire system operates only slightly above the ambientatmospheric pressure and the gas phase pressure and the gas phasepressure drop between successive gas headspaces will be extremely small.If the system uses a high purity oxygen feed gas as opposed toatmospheric air then the gas flow from the high purity oxygen generatorwill be directly connected to the feed gas stage as outlined above forthe case of air aeration. Sources of high purity oxygen for the presentinvention include all sources known to those skilled in the art andinclude: liquid oxygen tanks, on site pressure swing adsorption devices,vacuum swing adsorption facilities, and cryogenic production facilities.

It is a principal aspect of the present invention that an efficientgas-liquid contacting and sludge mixing device is contained in eachaerobic stage. Different types of gas-liquid contacting devices are wellknown to those skilled in the art and a specific gas-liquid contactingdevice design is not a requirement of the invention. Some well knowngas-liquid contacting devices include diffusers, surface aerators,submerged gas-liquid turbines, and jet aerators. The preferred means ofproviding the gas-liquid contacting and mixing within each stage is bythe use of surface aerators to effect the sludge circulation and mixingnecessary for efficient transfer of the oxygen containing gas into thesludge liquid. In practice, it is preferable that the aerating meansemployed be capable of achieving high oxygen transfer energy efficiencyand high mixing capability. U.S. Pat. No. 6,715,912 to J. McWhirter, etal. (hereby incorporated by reference) discloses such preferred surfaceaerator designs. As shown in FIG. 1 of that patent, the impeller isrotated on a vertical shaft mounted in a tank and is connected to apower source and gear reduction means. The impeller has a plurality ofblades mounted on the underside of a disc or disc-like surface. Eachblade has a multi-faceted or curved geometry ranging from vertical atthe point of attachment to the disc to partially inclined at the bottom.The blades are spaced circumferentially about the axis and are disposedradially or at acute angles to radial lines from the axis of rotation ofthe impeller. The lower portions of the blades, which are less inclinedor less vertical than the upper portions, are positioned below thestatic liquid surface.

When the impeller is rotated, the lower portion of the impeller bladepumps the sludge up onto the vertical portion of the blades where thesludge is discharged into a spray umbrella in a direction upwardly fromthe static liquid surface and outwardly away from the rotating impeller.In this manner, the impeller is able to transfer a high amount of oxygencontaining gas into the sludge while thoroughly mixing the entire bodyof sludge in the tank. In addition to, or alternatively, in lieu ofsurface aeration impellers as described above, the aerating means in oneor more of said tanks, tank segments or compartments comprises submergedmixing turbines for facilitating the transfer of oxygen containing gasinto the sludge.

In another embodiment of the invention, especially useful in smallerwastewater treatment plants, a single compartmented or segmented tank isused for the continuous multistage aerobic digestion system. In thiscase there is usually insufficient surface area in the tanks and/or tankcompartments to permit effective use of surface aeration impellers. Insuch cases, high oxygen transfer efficiency and liquid pumping rates canbe achieved through the use of a draft tube containing multiple axialflow impellers, as described in U.S. Pat. Nos. 5,972,661 and 6,464,384to J. McWhirter, et al. (hereby incorporated by reference). A draft tubesystem with multiple axial flow impellers alone can be used to effectmixing and circulation of oxygen containing gas throughout the tanksand/or tank compartments, however, a multiple axial flow impeller systemmay also be used in conjunction with a surface aeration impeller toobtain high percentage oxygen absorption.

The invention will now be described with reference to FIGS. 3 and 4.FIG. 3 shows one embodiment of a continuous multistage ATAD according tothe present invention having three stages and countercurrent gas flow.In FIG. 3, the incoming feed sludge stream 1 enters the first coveredand insulated (not shown) stage 8 and flows continuously through thesecond stage 9 and onto the third stage 10 prior to exiting as digestedsludge stream 4. The oxygen containing gas stream flows countercurrentto the sludge liquid and enters the system at the headspace of the thirdliquid stage as feed stream 5. The gas then flows continuously andsequentially through the headspaces of the second and first liquidstages before exiting the first liquid stage as exit gas stream 11. Theexit gas may be vented to the atmosphere or further processed for odorcontrol via means well-known to those skilled in the art. Gas-liquidcontacting and mixing is enhanced by a surface aerator 12 and optionalmixing impeller 13 in each stage.

FIG. 4 shows another embodiment of the invention having five aerobicsludge digestion gas-liquid contacting stages. Wastewater sludge fromprimary and/or secondary treatment facilities is first fed into athickening device 28 and then is sent to an optional holding tank 29.From the holding tank, the influent sludge is continuously sent to thefirst liquid stage 30 of the ATAD system. The liquid sludge then flowscontinuously through the remaining stages of the system 31, 32, 33, and34. Air or oxygen enriched gas feed enters the gas headspace of the lastliquid stage 34 and flows continuously through the remaining stages in acountercurrent flow manner. The oxygen containing gas exits the firstliquid stage 30 where it can be vented, recycled, or scrubbed. Processedliquid sludge exiting the system from the last stage 34 is optionallysent to an anaerobic digester 35 or into alternating batch reactors 36as previous discussed. An optional holding tank 37 is also shown. It isimportant to note that a major benefit of the present invention is theDual Digestion version of the invention. By operating a continuous flow,multistage thermophilic aerobic sludge digestion system according to theinvention prior to an existing anaerobic system the capacity of theanaerobic digester can be effectively at least doubled in addition toproducing pasteurized sludge. Also, it is noted that this figureillustrates an embodiment of the invention that uses a large firstliquid stage. As shown, the first stage is at least twice the size ofthe average of the other stages.

An important feature of the present continuous flow, multistage,thermophlic aerobic sludge digestion invention is its ability tosimultaneously pasteurize and stabilize sludge. The EPA terms suchdigested sludge a “Class A” biosolid and specifies in 40 CFR Part 503its requirements for achieving Class A status. The production of Class Abiosolids is becoming more important due to the EPA's much lessrestrictive rules for the beneficial use of such sludge (e.g.fertilization) and the public's perception of increased safety of ClassA sludge. The EPA Class A designation has stringent pathogen reductionand vector attraction reduction requirements. While the actualregulations are complex, one option for meeting the pasteurizationrequirement is to treat the sludge for a sufficient time at a highenough temperature as specified in the regulations. For example,according to the specified time-temperature requirements, it takes onlyabout 5 hours to pasteurize liquid sludge maintained at about 60° C.However, we note that 40 CFR 503 does not specifically addresscontinuous flow systems like the present invention. Hence we considerpasteurization to occur under conditions which give pathogen reductionin the continuous flow outlet stream that is equivalent to the specifiedbatch requirement conditions.

The EPA vector attraction reduction requirements are also complex andhave multiple options for achieving acceptable sludge stability. Thesimplest option for achieving sufficient vector attraction reduction isa reduction of volatile suspended solids content by 38% or more. We usethis definition of stabilized sludge in the present application.

In the systems of the present invention, pasteurization can be obtainedby one of three different methods. First, as stated above, sludge canhave a sufficient average residence time in the entire multistage systemat a sustained high enough temperature so that pathogen reductionequivalent to that which EPA regulations require for batch processes isensured. Second, the continuous flow multistage digester can exit intoalternating batch pasteurization tanks where the sludge is pasteurizedin a batch tank for a time sufficient to meet the specific 40 CFR 503time-temperature requirements. Two alternating batch pasteurizationtanks are required to ensure that the overall system can remaincontinuous. While one batch pasteurization tank is filling up the othertank is pasteurizing and emptying and vice-versa. The batchpasteurization tanks can be mixed, but generally need not be aerated.The digested sludge exiting from the aerated reactors remains continuousbecause one or the other of alternating batch pasteurization tanks isalways filling as a valve can simply switch the flow to the other tankafter it finishes pasteurizing and emptying. The third option forpasteurization relates to the DUAL DIGESTION™ version of the inventionwhere sludge exiting from the continuous multistage aerobic system ofthe invention flows directly or indirectly into an anaerobic digestionsystem. Sludge can have a sufficient residence time in the thermophlicaerobic sludge digestion system combined with an anaerobic digester toreach pasteurization according to the EPA regulations. The specificdesign of the anaerobic digestion system is not strictly a part of thepresent invention and any design known to those skilled in the art maybe used.

The overall temperature profile of the multiple stages of the continuousflow, sludge digestion invention is an important feature of the system.By “profile” is meant that each stage has a unique temperature that isrelatively constant throughout the stage and is relatively stable overtime (given a stable input stream). The individual gas-liquid stages areessentially completely mixed in both the gas phase and the liquid phaseand therefore have a constant and equal temperature in both phases aswell as a constant composition in both phases. The series oftemperatures for the stages is known as the system's overall temperatureprofile. For example, for a six stage system, a steadily increasingtemperature profile might have the following temperatures in stages 1–6respectively: 45-55-60-63-65-66° C. Likewise, an increasing temperatureprofile followed by a decreasing profile in the same multistage,continuous flow system might have the following temperatures in stages1–6: 45-55-63-60-48-40° C. This temperature profile which might betermed as a “humped” profile having at least one mesophilic stage at thebeginning and end of the system and 1, 2 or 3 intermediate thermophilicstages is a preferred embodiment of the invention, especially for thosesystems using countercurrent air flow. Alternatively, just the last oneor two stages operates in the mesophilic temperature range. It is knownthat unpleasant odors exist in both the gas and liquid streams ofaerobic sludge digestion systems operating only under thermophilicconditions. These unpleasant odors can be greatly reduced or eliminatedby putting the exiting gas and sludge liquid streams through one or morestages operating in the mesophilic temperature range prior to exitingthe multistage system.

The steadily increasing sludge temperature profile is often preferredwhen the last aerobic stage of the multistage system exits toalternating batch pasteurization tanks or to an anaerobic digester.Having a mesophilic last liquid stage (and preferably both first andlast stages) is generally preferred over other designs. The lastmesophilic stage can be provided by a number of means. First, with anair system, making the last sludge liquid stage the first or second gascontacting stage will typically cool the last liquid stage down to themesophilic temperature range due to the substantial evaporative heatlosses into the high flowing air stream. It is an important element ofthis invention, however, that most of the heat absorbed into the feedair stream in the later liquid stages is recovered via directcondensation into the incoming liquid sludge stream as the air streamexits a cooler stage, such as the first sludge liquid stage in themultistage system. This would be the case in a countercurrent air flowsystem. Second, a mesophilic last liquid stage can be provided by activecooling such as by the use of internal heat exchangers or cooling coilsinside the tanks. However, this is a much more expensive option and isnot preferred in the present invention. Finally, in a primarily highpurity oxygen system the last one or two stages can be separatelyaerated with air to provide a cooling effect through evaporative heatlosses. This may be especially useful in high purity oxygen systemssince evaporative heat losses by using countercurrent high purity oxygengas flow alone will be minimal because of the substantially lower totalgas flow rates. It is also noted that one or more mesophilic stages maybe operated after alternating batch pasteurization tanks as discussedabove.

Although not shown in any figure, various control schemes may beemployed to control the temperature and/or digestion rate of the sludgein any particular stage. One example is the use of variable frequencydrives (VFDs) on the surface aerators. VFDs can be used to modulate thespeed of rotation and hence the oxygen transfer rate into each tankand/or tank segment based on the rate of dissolution of oxygencontaining gas into the sludge and/or the liquid phase temperature. Inthis way, oxygen transfer can be precisely controlled in accordance witha feedback control system. It will be understood that while specifictank, pump, piping and controller arrangements are described herein, itis foreseen that known systems for directing and controlling the flow ofwastewater and sludge through various components of a wastewatertreatment plant may be employed.

EXAMPLES

The invention will now be described and illustrated by the followingexamples. The examples were obtained by fundamentally rigorousmathematical modeling of the continuous flow, multistage aerobic sludgepasteurization and stabilization systems of the invention. Thesimulation program uses complete multistage mass and energy balancescoupled with detailed multicomponent mass transfer and kinetic reactioncalculations to accurately model a very broad range of design conditionsand performance characteristics. Input variables include: number andsize/dimensions of stages; feed sludge flow rate and temperature; feedsludge TSS, VSS, and BVSS concentrations; gas feed flow rate andcomposition, and gas flow staging order; and the performance parametersof the gas-liquid contacting units including SAE, alpha factor, betafactor, and optionally power input. The power inputs of the gas-liquidcontacting units can be directly specified or the required powerrequirements are automatically determined to maintain positive dissolvedoxygen levels in the individual stages.

Example 1 Ten Stage System with Countercurrent Flow Air Aeration Gas

The total number of stages is 10 with an overall volume of 212,207gallons which provided a total sludge residence time of 4.13 days. Theinput sludge stream had a flow rate of 51,430 gallons per day at atemperature of 12° C. The TSS content of the sludge was 2.5 wt %, theVSS/TSS ratio was 0.84 and the BVSS/VSS ratio was 0.55. The air feedrate corresponded to a total contained oxygen feed rate of 12 tons perday of oxygen gas and the gas staging flow was completely countercurrentrelative to the liquid staging. The surface aerator gas-liquidcontacting devices in each stage was assumed to have an SAE of 3.5, analpha factor of 0.7, and a beta factor of 0.92.

The model simulation performance results showed a total system BVSSreduction of 63.1%, overall oxygen utilization efficiency of 20.9%, andtotal surface aerator shaft horsepower requirements of 115 HP.Additionally, the stages had a temperature profile and power input asgiven in the table below. The table also shows the sludge residence timein each stage (which is solely determined by the volume of each stage)and gives an indication of the degree of pasteurization occurring ineach stage. The pasteurization indicator level is the ratio of theresidence time in that stage divided by the time required forpasteurization at that stage temperature according to the EPA'stime-temperature relationship given in 40 CFR 503. A stagepasteurization ratio of less than 1.0 means that the residence time ismore than sufficient for complete pasteurization at that stage residencetime and the lower the ratio, the more pasteurization will be obtained.It is noted that the EPA's time-temperature relationships are notintended for continuous flow systems. However, given the ability ofeffectively designed multistage systems to closely approximate plug-flowor batch system residence time flow distribution characteristics, webelieve a multistage system with a number of stages having apasteurization ratio of less than 1.0 is a very good indicator of thehigh level of sludge pasteurization achieved by the present invention.

TABLE 1 Example 1 Model Simulation Performance Results Stage # 1 2 3 4 56 7 8 9 10 Sludge Stage Residence 0.20 0.38 0.38 0.38 0.38 0.38 0.560.73 0.38 0.38 Time (days) 26.0 50.2 63.5 67.2 67.6 66.9 65.6 62.6 56.346.0 Stage Temp (° C.) Aerator Shaft HP 5.2 17.4 17.2 10.3 7.9 9.6 15.618.4 7.5 5.9 Stage Pasteurization Ratio — 12.4 0.17 0.05 0.05 0.06 0.060.12 1.77 48.8

This example clearly shows that the continuous multistage air ATADsystem according to the invention readily achieves high sludgethermophilic digestion temperatures, a high level of BVSS reduction anda high degree of sludge pasteurization as indicated by thepasteurization ratio being substantially less than 1.0 in 6 successivestages out of the total 9 stage system. It is noted that these resultsare achieved with a feed sludge having a very low BVSS content and at avery low sludge feed temperature which represents extremely challengingprocess design parameters.

Example 2 Nine Stage System with Countercurrent Flow Air Aeration Gas

This simulation is similar to Example 1 except that the air feed rate isreduced to 9 tons per day of contained oxygen gas and the sludge feedwas thickened to 3.4 wt % TSS at a total flow rate of 37,816 gallons perday. Additionally, the total number of stages was reduced to 9 and theoverall sludge residence time was increased to 5.12 days. Simulationresults show a total system BVSS reduction of 67.2%, overall oxygenutilization efficiency of 22.2%, and total surface aerator shafthorsepower requirement of 166 HP as shown in the table below.

TABLE 2 Example 2 Model Simulation Performance Results Stage # 1 2 3 4 56 7 8 9 Sludge Stage Residence Time 0.51 0.51 0.51 0.51 0.51 0.51 0.770.77 0.51 (days) Stage Temp (° C.) 55.0 67.1 68.7 68.5 67.6 66.0 63.157.0 45.0 Aerator Shaft HP 36.3 21.6 10.4 11.8 15.5 18.3 22.7 19.2 10.2Stage Pasteurization Ratio 1.94 0.04 0.02 0.03 0.03 0.06 0.10 0.68 49.4

This example clearly shows excellent sludge pasteurization and high BVSSreduction. It also highlights a couple of important system performancecharacteristics compared with Example 1. First, reducing the air flowtends to increase the surface aerator horsepower requirements. Second,increasing the sludge residence time tends to increase the overallsludge digestion temperatures and increase the overall BVSS reduction.

Example 3 Nine Stage System with Countercurrent Flow Air Aeration Gas

This example was the same as Example 2 but illustrates how the inventivesystem handles a medium solids content sludge (3.5% TSS) at a very lowresidence time (2.33 days). This design achieves a total BVSS conversionof 47.7%, total surface aerator shaft horsepower requirement of 112 HP,and total oxygen utilization of 15.8%. Other results are as follows:

TABLE 3 Example 3 Model Simulation Performance Results Stage # 1 2 3 4 56 7 8 9 Sludge Stage Residence Time 0.26 0.26 0.26 0.26 0.26 0.26 0.260.26 0.26 (days) Stage Temp (° C.) 46.0 62.8 66.3 65.9 64.3 61.7 57.851.9 41.5 Aerator Shaft HP 15.7 16.5 12.8 12.2 13.7 12.4 11.2 9.6 7.7Stage Pasteurization Ratio 70.7 0.32 0.10 0.11 0.19 0.45 1.56 10.41 —

This example clearly shows exceptional pasteurization performance at avery low total sludge residence time. However, the low residence time isnot sufficient to obtain a high degree of BVSS conversion and thus thisdesign would not be particularly useful for producing a Class A sludge.However, this system design is well suited to provide a pasteurized feedinto an anaerobic digester as it is generally preferred to minimize theamount of BVSS reduction that occurs prior to the anaerobic digester.Anaerobic digesters prefer high BVSS content sludge since the BVSS isthe principle resource for methane production.

Example 4 Two Stage System with Countercurrent Flow Air Aeration Gas

In this two stage continuous flow air system design, the feed sludge hasa temperature of 10° C. and a flow rate of 23,880 gallons per day at aTSS of 4.7 wt %, a VSS/TSS ratio of 0.80 and a BVSS/VSS ratio of 0.70.The feed air stream has a contained oxygen content of 8 tons per day.The total volume of the two stages was 55,636 gallons (10,473 gallons inthe first stage and 45,164 gallons in the second stage) giving a totalsystem residence time of 2.33 days. The surface aerator horsepowers werefixed at 15 HP in the first stage and 60 HP in the second stage.Simulation results show a total BVSS reduction of 44.2% and an oxygenutilization of 23.2%. Additional results are shown in the table below.

TABLE 4 Example 4 Model Simulation Performance Results Stage # 1 2Sludge Stage Residence Time (days) 0.44 1.89 Stage Temp (° C.) 46.3 60.8Aerator Shaft HP 15.0 60.0 Stage Pasteurization Ratio 37.7 0.08

This example demonstrates the ability of the invention to produce apasteurized sludge in only two stages and only 2.33 days of totalresidence time. The total system BVSS reduction is 44.2% so this designdoes not adequately stabilize the sludge for the Class A designation byitself, but the exit sludge would be very useful for the feed to asubsequent anaerobic digestion system.

Example 5 Six Stage System with High Purity Oxygen Aeration with PartialCocurrent and Countercurrent Gas Flow

The total number of stages in this design is 6 with an overall volume of110,801 gallons which provides a total sludge residence time of 2.47days. The input sludge stream has a flow rate of about 45,000 gallonsper day at a temperature of 10° C. The TSS content of the sludge is 5.0wt %, the VSS/TSS ratio is 0.80 and the BVSS/NVSS ratio is 0.60. Thehigh purity oxygen (90% by volume O₂) feed rate corresponds to a totalcontained oxygen feed rate of 5 tons per day of oxygen gas. The highpurity oxygen gas is fed into stage 3 and then flows into stages 4, 5, 6and 2 in that order and exits from stage 1 and is thus partiallycocurrent and partially countercurrent with respect to the sludge liquidflow. The surface aerator gas-liquid contacting devices in each stagehave an SAE of 4, an alpha factor of 0.7, and a beta factor of 0.92.

This system design achieves a total BVSS reduction of 43.6%, an overalloxygen utilization efficiency of 62.8%, and requires a total surfaceaerator shaft horsepower of 39.8 HP. Additionally, the stages have atemperature profile and power input requirement as given in the tablebelow.

TABLE 5 Example 5 Model Simulation Performance Results Stage # 1 2 3 4 56 Sludge Stage Residence 0.62 0.62 0.31 0.31 0.31 0.31 Time (days) StageTemp (° C.) 20.1 37.8 44.5 52.4 59.4 64.0 Aerator Shaft HP 10.9 12.8 3.64.2 4.5 3.8 Stage Pasteurization Ratio — — 96.8 7.45 0.79 0.18

This example demonstrates a continuous multistage high purity oxygenATAD system having 6 stages according to the invention. This designwould also be a good choice for producing a pasteurized sludge forfeeding into an anaerobic digester as high temperatures are achieved inmultiple stages and the BVSS reduction is not too high. This examplealso illustrates the usefulness of the invention for higher solidscontent sludges and also the usefulness of having larger first and/orsecond stages to more rapidly bring the sludge up to thermophilictemperatures. The advantage of a partially cocurrent and partiallycountercurrent gas flow staging is also demonstrated in this design.

Example 6 Three Stage System with High Purity Oxygen Aeration withPartial Cocurrent and Countercurrent Gas Flow

This example provides another demonstration of a design suitable for usein a Dual Digestion™ system. This example uses 3 stages and high purityoxygen (90%) at a contained oxygen gas feed rate of 3 tons per day. Thesludge feed has a temperature of 10° C. at a flow rate of about 45,000gallons per day and the total system volume is 130,062 gallons. The gasfeed order was 2, 3, 1 and the surface aerator performance parametersare the same as the previous example. The design results in a BVSSreduction of 49.7% and a total surface aerator horsepower of 80.5 HPwith an oxygen utilization efficiency of 91.8%. Results are given in thetable below.

TABLE 6 Example 6 Model Simulation Results Stage # 1 2 3 Sludge StageResidence Time (days) 0.97 0.97 0.97 Stage Temp (° C.) 29.3 46.5 60.8Aerator Shaft HP 47.9 11.7 21.0 Stage Pasteurization Ratio — 16.2 0.16

This example shows the potential of the present invention forpasteurization using only 3 stages and less than 3 days total systemsludge residence time. It is noted that the total surface aerator powerrequirement is rather high, but this could be decreased easily byincreasing the high purity oxygen feed rate and thus lowering theoverall system oxygen utilization efficiency.

These examples dramatically demonstrate the enhanced performance of thepresent aerobic sludge digestion invention. The present invention is aneffective and environmentally responsible means of achieving both sludgepasteurization and stabilization and producing digested sludge thatmeets the current regulations for disinfection and disposal ofagricultural, municipal and industrial wastewater sludges on land andunderground. The system is autothermally operated in the thermophilictemperature range that achieves aerobic stabilization of the sludge atconsiderably lower residence times. As a result, the inventionsignificantly lowers both capital and operating costs. The systemeffectively separates wastewater sludge into low residual water andsafely disposable organic matter, which allows the water to be recycledback into the environment (or reused) and the organic matter to bereduced, biologically stabilized, and either recycled (e.g. as anagricultural fertilizer) or disposed of safely (e.g. in a landfill).

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various alterations in form and detail maybe made therein without departing from the spirit and scope of theinvention. In particular, while the invention illustrated by theexamples and figures shows a specific size and number of the reactorstages and specific surface aerator design and performance parameters,these and other design features may be widely varied and are not at alllimited by the preferred embodiments described herein.

1. A continuous flow, enclosed and heat insulated, multistage gas-liquidcontacting system for the thermophilic aerobic digestion, pasteurizationand stabilization of concentrated wastewater sludge, wherein: a) eachgas-liquid contacting stage contains a separate sludge liquid volume, anenclosed separate gas headspace above the sludge liquid and a gas-liquidcontacting device to contact the sludge liquid with the gas in theheadspace; b) the sludge liquid flows from the feed liquid stageserially through the entire multistage system; c) the gas feed to themultistage system contains at least 20 volume percent oxygen gas andflows directly from the feed gas headspace to one or more other gasheadspaces in the multistage system in a predetermined sequence; and d)the sludge liquid temperature reaches a minimum of 58° C. in at leastone stage at a sufficient residence time to ensure pasteurization. 2.The continuous flow thermophilic aerobic sludge digestion systemaccording to claim 1 wherein no external heating or cooling is appliedto the system.
 3. The continuous flow thermophilic aerobic sludgedigestion system according to claim 2 wherein no heat exchange equipmentis employed.
 4. The continuous flow thermophilic aerobic sludgedigestion system according to claim 1 wherein the flow of gas throughthe multistage system is countercurrent with respect to the flow of thestaged liquid.
 5. The continuous flow thermophilic aerobic sludgedigestion system according to claim 1 wherein the flow of gas throughthe multistage system is cocurrent with respect to the flow of thestaged liquid.
 6. The continuous flow thermophilic aerobic sludgedigestion system according to claim 1 wherein the flow of gas throughthe multistage system is neither completely cocurrent nor completelycountercurrent with respect to the flow of the staged liquid.
 7. Thecontinuous flow thermophilic aerobic sludge digestion system accordingto claim 1 wherein the sludge liquid residence times in the individualstages are not equal.
 8. The continuous flow thermophilic aerobic sludgedigestion system according to claim 1 having at least three gas-liquidcontacting stages.
 9. The continuous flow thermophilic aerobic sludgedigestion system according to claim 1 having from 5–10 gas-liquidcontacting stages.
 10. The continuous flow thermophilic aerobic sludgedigestion system according to claim 1 wherein sludge temperatures withinthe system are controlled by one or more techniques selected from thegroup consisting of: controlling solids concentration of the sludgefeed, controlling the oxygen content of the feed gas, controlling thefeed gas flow rate, and controlling the temperature of the sludge feedstream.
 11. The continuous flow thermophilic aerobic sludge digestionsystem according to claim 1 wherein said gas-liquid contacting devicesare surface aerators.
 12. The continuous flow thermophilic aerobicsludge digestion system according to claim 11 wherein said surfaceaerators provide a high rate of oxygen transfer from the gas phase intothe liquid phase and effectively mix the liquid sludge contents of thestages.
 13. The continuous flow thermophilic aerobic sludge digestionsystem according to claim 11 wherein the surface aerator gas-liquidcontacting devices additionally include a bottom mixing impeller in oneor more stages.
 14. The continuous flow thermophilic aerobic sludgedigestion system according to claim 1 wherein the oxygen containing gasis air.
 15. The continuous flow thermophilic aerobic sludge digestionsystem according to claim 14 wherein the sludge liquid residence time inone or more of the final stages is greater than that in one or moreprior stages.
 16. The continuous flow thermophilic aerobic sludgedigestion system according to claim 14 wherein the sludge liquidresidence time in one or more of the initial stages and in one or moreof the final stages is greater than that in one or more of the otherstages.
 17. The continuous flow thermophilic aerobic sludge digestionsystem according to claim 1 wherein the oxygen containing gas is oxygenenriched gas containing greater than 21 volume percent oxygen.
 18. Thecontinuous flow thermophilic aerobic sludge digestion system accordingto claim 17 wherein the oxygen containing gas is high-purity oxygen gascontaining greater than 80 volume percent oxygen.
 19. The continuousflow thermophilic aerobic sludge digestion system according to claim 18wherein the sludge liquid residence time in one or more of the initialstages is greater than that in one or more subsequent stages.
 20. Thecontinuous flow thermophilic aerobic sludge digestion system accordingto claim 1 wherein the high temperature concentrated sludge ismaintained at a temperature of at least 60° C. in at least two stages.21. The continuous flow thermophilic aerobic sludge digestion systemaccording to claim 1 wherein the total residence time in the entiremultistage system is sufficient to reduce volatile suspended solids(VSS) by at least 38 weight percent.
 22. The continuous flowthermophilic aerobic sludge digestion system according to claim 21wherein the total residence time in the entire multistage system is lessthan 5 days.
 23. The continuous flow thermophilic aerobic sludgedigestion system according to claim 1 wherein the temperature profile ofthe multiple stages increases with the liquid flow such that the firstliquid stage has the lowest temperature and the last liquid stage hasthe highest temperature.
 24. The continuous flow thermophilic aerobicsludge digestion system according to claim 23 wherein the last andhighest temperature liquid stage achieves a temperature greater thanabout 60° C. and is discharged into alternating batch pasteurizationtanks wherein the sludge liquid is held in batch mode at a sufficienttime and temperature to ensure pasteurization.
 25. The continuous flowthermophilic aerobic sludge digestion system according to claim 1 havingmore than 1 gas feed stream and at least one feed stream is air.
 26. Thecontinuous flow thermophilic aerobic sludge digestion system accordingto claim 1 having at least three stages and wherein the temperatureprofile of the multiple stages is such that the highest temperatureliquid stage is not the first or last stage and at least the first andlast stages are operated in the mesophilic temperature range less than50° C.
 27. The continuous flow thermophilic aerobic sludge digestionsystem according to claim 1 for producing a pasteurized and stabilizedClass A sludge wherein: i) the oxygen containing gas is air; ii) thesystem contains from 4–10 total gas-liquid contacting stages; iii) atleast the first and last stages are operated in the mesophilictemperature range; iv) at least two adjacent stages are operated in thethermophilic temperature range; and v) the gas staging order is at leastpartially countercurrent.
 28. The continuous flow thermophilic aerobicsludge digestion system for producing a pasteurized and stabilized ClassA sludge according to claim 27 wherein the total number of stages is atleast five and wherein at least three adjacent intermediate stages areabove 60° C.
 29. The continuous flow thermophilic aerobic sludgedigestion system for producing a pasteurized and stabilized Class Asludge according to claim 27 wherein the temperature of the system iscontrolled by either controlling the solids concentration of the sludgefeed stream or by controlling the air feed flow rate.